SHALLOW LAKES Reaction of large and shallow lakes Peipsi and Vo ˜ rtsja ¨rv to the changes of nutrient loading Tiina No ˜ ges ® Arvo Ja ¨ rvet ® Anu Kisand ® Reet Laugaste ® Enn Loigu ® Boris Skakalski ® Peeter No ˜ ges Ó Springer Science+Business Media B.V. 2007 Abstract More than 20-year monitoring of Estonian rivers reveals that the loading of nitro- gen to large shallow lakes Peipsi (3,555 km 2 , mean depth 7.1 m) and Vo ˜ rtsja ¨rv (270 km 2 , mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Vo ˜ rtsja ¨rv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catch- ment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Vo ˜ rtsja ¨rv the N/P ratio is higher and the ecosystem is more stable although the share of N 2 -fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish- kills during water-blooms are a straightforward consequence of reduced nitrogen level at remain- ing high phosphorus level while in L. Vo ˜ rtsja ¨rv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Vo ˜ rtsja ¨rv nutrient loading has Guest editors: R. D. Gulati, E. Lammens, N. De Pauw & E. Van Donk Shallow lakes in a changing world T. No ˜ges (&) Á A. Kisand Á R. Laugaste Á P. No ˜ ges Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Rannu, Tartumaa 61101, Estonia e-mail: tiina.noges@emu.ee A. Ja ¨ rvet Institute of Geography, University of Tartu, Vanemuise 46, Tartu 51014, Estonia E. Loigu Department of Environmental Engineering, Tallinn Technical University, Ehitajate tee 5, Tallinn 19086, Estonia B. Skakalski Russian Hydrometeorological University, Malookhtinsky avenue 98, Saint Petersburg 195196, Russian Federation P. No ˜ ges Institute for Environment and Sustainability, European Commission – DG Joint Research Centre, Ispra, VA 21020, Italy 123 Hydrobiologia (2007) 584:253–264 DOI 10.1007/s10750-007-0603-z